System and method for cryogenic cooling of a process stream with enhanced recovery of refrigeration

ABSTRACT

A system and method for improved cryogenic cooling of process gases is provided. The disclosed system and method provides for the cryogenic cooling of a silane and hydrogen gas process stream during the manufacture of polysilicon with concurrent recovery of refrigeration capacity from the vaporized nitrogen as well as the recovery of refrigeration capacity from the cold hydrogen gas stream. The improved cryogenic cooling system and method reduces the overall consumption of liquid nitrogen without sacrificing cooling performance of the cryogenic cooling of the silane and hydrogen gas process stream.

CLAIM OF PRIORITY

The present application is a National Stage Entry of PCT/US2011/059704filed Nov. 4, 2011, which claims priority from Provisional Application61/414,145 filed Nov. 16, 2010. No new matter has been added.

FIELD OF THE INVENTION

The present system and method relates to improved cryogenic cooling ofprocess gases, and more particularly to a system and method for thecryogenic cooling of a process stream with concurrent recovery ofrefrigeration from both the spent cryogen and the cold process stream.

BACKGROUND

Cryogens such as liquid nitrogen are often used to cool process gasstreams to very low temperatures in many manufacturing processes.Example of using cryogenic cooling systems in manufacturing processesinclude: cooling reactors of exothermic reactions; chemical process thatrequire low temperatures to improve product selectivity; recovercondensable products from a gaseous stream; and solution crystallizationto purify products from a mixture of dissolved solids.

The economics of using cryogenic liquid nitrogen for cooling in manymanufacturing processes depends heavily on the ability to recover therefrigeration from the spent nitrogen and from the cooled process gasstream. Ideally, the temperature of the spent nitrogen and the processgas stream should be warmed to about room temperatures or the finaldesirable downstream operating temperatures without external heating ortemperature adjustments. Conventionally, the spent nitrogen is anitrogen gas stream that is directed to an economizer where it is usedto cool against the incoming process stream.

FIG. 1 shows an example of a conventional vapor phase product recoveryprocess. As seen therein, a warm process stream (12) is first pre-cooledin an economizer (13) cooled by a cold nitrogen gas stream (26). Thepartly cooled process stream (14) is then directed to a primary heatexchanger (15) where the liquid nitrogen (24) is vaporized and cools theprocess stream (14) to it final temperature. The resulting processstream (16) is then directed to a phase separator (17) while thevaporized cold nitrogen gas (26) is routed back to the economizer (13)to cool the incoming warm process stream (12). The final product (20) iscondensed in the phase separator (17) and recovered as a liquid with theresidual cold process gas stream (30), without the condensed product, isvented or otherwise discarded or recycled within the plant. Most, if notall of the refrigeration capacity in the cold process gas stream islost.

In other prior art systems, the refrigeration capacity from the cold gasprocess stream is recovered but the refrigeration capacity from the coldnitrogen gas stream exiting the primary heat exchanger is lost. FIG. 2is a schematic illustration of this alternate arrangement for recoveringsome of the refrigeration from a cryogen gas stream used to produce acondensable product. As seen therein, a warm process stream (12) isfirst pre-cooled in an economizer (13) cooled by the cold process gasstream (30) exiting the phase separator (17). The partly cooled orpre-chilled process stream (14) is then directed to the primary heatexchanger (15) where the liquid nitrogen (24) is vaporized and cools theprocess stream (14) to it final temperature. The resulting cooledprocess stream (16) is then directed to a phase separator (17) while thevaporized cold nitrogen gas (26) is vented. The desired product (20) iscondensed in the phase separator (17) and recovered as a liquid with theresidual cold process gas stream (30), without the condensed product, isrecycled back to the economizer (13) to pre-chill the incoming warmprocess gas (12) after which it is vented or otherwise discarded.

Using these conventional cryogenic cooling processes, a substantialamount of refrigeration capacity is not recovered. If either of thespent nitrogen gas stream or the cold process stream, without thecondensed product, is used to pre-chill the incoming warm process gasstream, there will be little thermodynamic driving force for the othercold stream to transfer additional refrigeration values to thepre-chilled process stream. Refrigeration capacity recovery is even moredifficult if any phase transformations of the cryogen or the processstream are involved. In most industrial applications, it is estimatedthat about half of the refrigeration capacity cannot be recovered by theabove-described conventional methods as the refrigeration capacity ofthe liquid nitrogen is dissipated in the form of latent heat.

What is needed, therefore, is a cryogenic cooling system that minimizesthe use of cryogens and also maximizes the recovery of the availablerefrigeration capacity within the cooling and purification processes.

SUMMARY OF THE INVENTION

The present invention may be charterized as a method for cryogeniccooling of a process stream comprising the steps of: (a) pre-chilling aninfluent of warm process gas in an economizer; (b) cooling thepre-chilled process gas with a cryogen in a cryogenic heat exchanger toa prescribed final temperature; (c) separating the cooled process gas atthe prescribed final temperature into a condensable product and a coldspent process gas; (d) recycling the cold spent process gas to theeconomizer to pre-chill the influent of warm process gas; (e) forciblydirecting a portion of the used process gas recycled to the economizerto an auxiliary heat exchanger; and (f) directing the spent cryogen fromthe cryogenic heat exchanger to the auxiliary heat exchanger to re-coolthe used process gas. Using this method, the excess refrigerationcapacity of the cold spent process gas is directly transferred to theinfluent warm process gas flowing through the economizer and the excessrefrigeration capacity of the spent cryogen is indirectly transferred tothe influent warm process gas flowing through the economizer. This, inturn minimizes the amount of cryogen needed to cool the pre-chilledprocess gas in the cryogenic heat exchanger to the prescribed finaltemperature.

The present invention may also be characterized as a cryogenic coolingsystem comprising: (i) a process stream; (ii) a source of cryogen; (iii)a cryogenic heat exchanger for cooling the process stream using thecryogen; (iv) a phase separator adapted for separating the cooledprocess stream into a condensable product and a cold spent process gas;(v) an economizer for pre-chilling the process stream with the coldspent process gas; (vi) a first recycle conduit coupling the outlet ofthe phase separator to the economizer to direct the cold spent processgas from the phase separator to the economizer to pre-chill the processstream; (vii) a second heat exchanger coupled to the cryogenic heatexchanger and adapted for using spent cryogen from the cryogenic heatexchanger to cool a stream of used process gas recycled to theeconomizer; (viii) a second recycle conduit coupling the outlet of theeconomizer through the second heat exchanger and to either the firstrecycle conduit or the inlet of the economizer to pre-chill the processstream; (ix) a blower disposed in operative association with the secondrecycle conduit to forcibly drive the used process gas from the outletof the economizer through the second heat exchanger to either the firstrecycle conduit or inlet of the economizer. The excess refrigerationcapacity of the spent cryogen from the cryogenic heat exchanger istransferred first to the used process gas flowing through the secondheat exchanger and subsequently to the influent process stream flowingthrough the economizer. Similarly, the excess refrigeration capacity ofthe cold spent process gas exiting the phase separator is transferreddirectly to the influent process stream flowing through the economizer.

Still further, the present invention may be characterized as an improvedcryogenic cooling system comprising a cryogenic heat exchanger forcooling an influent process stream; a phase separator downstream of thecryogenic heat exchanger for separating the cooled process stream into acondensable product and a cold spent process gas; and an economizer forpre-chilling the process stream upstream of the cryogenic heatexchanger, the improvement further comprising: (i) a second heatexchanger coupled to the cryogenic heat exchanger; (ii) a first recycleconduit coupling the outlet of the phase separator to the economizer todirect the cold spent process gas from the phase separator to theeconomizer; (iii) a second recycle conduit coupling the outlet of theeconomizer through the second heat exchanger and to either the firstrecycle conduit or the inlet of the economizer; and (iv) a blowerdisposed in operative association with the second recycle conduit toforcibly drive process gas from the outlet of the economizer through thesecond heat exchanger and to either the first recycle conduit or theinlet of the economizer. Using the above-described improvement, theexcess refrigeration capacity of the spent cryogen from the cryogenicheat exchanger is transferred first to the used process gas flowingthrough the second heat exchanger and subsequently to the influentprocess stream flowing through the economizer. Similarly, the excessrefrigeration capacity of the cold spent process gas exiting the phaseseparator is transferred directly to the influent process stream flowingthrough the economizer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following, more detaileddescription thereof, presented in conjunction with the followingdrawings, wherein:

FIG. 1 is a schematic illustration of a cryogenic cooling system showingthe conventional process of recovering refrigeration from a spentcryogen stream using an economizer;

FIG. 2 is a schematic illustration of a cryogenic cooling system showingthe conventional process of recovering refrigeration from a cold processstream using an economizer; and

FIG. 3 is a schematic illustration of the present cryogenic coolingsystem depicting an improved system and process for concurrentlyrecovering refrigeration from both a cold process stream and a spentcryogen stream.

DETAILED DESCRIPTION

Turning to FIG. 3, there is shown a schematic illustration of thepresent cryogenic cooling system depicting the improved system andmethod for concurrently recovering refrigeration from both a coldprocess stream and the spent cryogen using an economizer. As is wellknown in this art, eeconomizers are mechanical devices intended toreduce energy consumption during a process, or to perform usefulfunctions like pre-chilling a process gas.

The present system and method uses both direct refrigeration recovery ofthe cold spent process gas to the influent warm process gas via theeconomizer as well as indirect refrigeration recovery of the spentcryogen to the recycled process gas in an auxiliary heat exchanger andsubsequently to the influent warm process gas via the economizer. Thisdual refrigeration recovery approach improves the performance of theeconomizer which in turn reduces the amount of cryogen needed to achievethe desired or prescribed final temperature for product separation.

In the illustrated embodiment, the influent warm process stream (52) iscooled in a multi-step process using an upstream economizer (53) and aprimary cryogenic heat exchanger (55). The influent warm process stream(52) is first cooled to a pre-chilled process stream (54) at aprescribed intermediate temperature and then to a fully cooled processstream (56) at the prescribed final temperature. The fully-cooledprocess stream (56) is then directed to a phase separator (57) where theprocess stream is separated into a liquid product (60) and a cold spentprocess gas (80). It should be noted however, that this cooling of theinfluent warm process stream (52) may also be accomplished in a seriesof cooling steps using a plurality of economizers or heat exchangers.

The cryogen source used in the cryogenic heat exchanger (55) ispreferably liquid nitrogen (64) to cool the pre-chilled process gasprior to its separation. The amount and flow of liquid nitrogen requiredto attain the prescribed final temperature is dependent, in part on theprocess stream being cooled and the intermediate temperature attained bythe economizer. The spent nitrogen (66), in gaseous form at about isthen directed to a second or auxiliary heat exchanger (59) where itcools the recycled, used process gas (86). The gaseous nitrogen (68)exiting the auxiliary heat exchanger (59) is subsequently vented orreleased to the atmosphere. This multi-step use of the cryogenicnitrogen recovers and utilizes a significant portion of the availablerefrigeration capacity of the cryogen.

The separation of the fully cooled process stream (56) into acondensable product and a cold spent process gas occurs at the finalprescribed temperature. The cold spent process gas (80) is thenrecirculated to the economizer (53) for cooling the influent warmprocess stream (52) to an intermediate process stream (54). The nowwarm, used process gas (84) is divided into two streams. One portion ofthe warm, used process gas (85) is vented or directed elsewhere in theplant whereas the second portion of the warm, used process gas (86) isrecycled to the second or auxiliary heat exchanger (59) using an blower(87). This warm, used process gas (86) is re-cooled in the second heatexchanger (59) using the spent nitrogen gas (66). The re-cooled, usedprocess gas (88) is then combined with the cold, spent process gas (80)from the separator (57). The combined stream (82) is directed to theeconomizer to cool the influent warm process stream (52).

The excess refrigeration capacity of the spent nitrogen (66) from thecryogenic heat exchanger (55) is transferred indirectly to the influentwarm process stream (52) by first transferring the refrigerationcapacity to the recycled, used process gas (86) flowing through thesecond heat exchanger (59) which, in turn, is subsequently used to coolthe influent warm process stream (52) flowing through the economizer(53). In addition, the excess refrigeration capacity of the cold spentprocess gas (80) exiting the phase separator (57) is transferreddirectly to the influent warm process stream (52) flowing through theeconomizer (53).

By using both direct refrigeration recovery and indirect refrigerationrecovery, the influent warm process gas is pre-chilled to a lowertemperature. This, in-turn, reduces the amount of cryogen needed in thecryogenic heat exchanger to obtain the desired or prescribed finaltemperature for separation. The reduction in cryogen use lowers theoperating costs associated with the cryogenic cooling system andprocess.

INDUSTRIAL APPLICABILITY

The present cryogenic cooling system and method is useful in manyindustrial applications, including for example in the manufacture ofpolysilicon using the Ethyl Corp developed fluidized bed process. Insuch polysilicon production application, the influent or feed processstream (52) is a gaseous stream of silane in hydrogen which is cooled ina multi-step process and the resulting cooled process stream (56) issubsequently separated into liquid silane (60) and hydrogen gas (80).The cooling and separation of the influent or feed process stream (52)is accomplished using a first economizer (53) followed by a cryogenicheat exchanger (55) and then the separator (57). It should be notedhowever, that this initial cooling of the influent or feed processstream can be accomplished in a series of steps using one or moreeconomizers or heat exchangers.

The cryogen source used in the cryogenic heat exchanger (55) ispreferably liquid nitrogen (64) at −179° C. to cool the process gasprior to its separation. The spent nitrogen (66), in gaseous form atabout −164° C., is then directed to a second heat exchanger (59) whereit cools the warm, spent process gas (86) (i.e. hydrogen gas). Thegaseous nitrogen (68) at about 14° C. is subsequently vented or releasedto the atmosphere. This multi-step use of the cryogenic nitrogenrecovers and utilizes a significant portion of the availablerefrigeration capacity of the cryogen.

The separation of the cooled process stream (56) of silane (SiH₄) andhydrogen gas (H₂) produces liquid silane at −173° C. and hydrogen gas atabout −172° C. The cooled hydrogen gas (80) is then recirculated to theeconomizer (53) for cooling the 25° C. influent or feed process stream(52) to an intermediate pre-chilled process stream (54). The warm, spenthydrogen gas (84) at about 11° C. is divided into two streams. Oneportion of the warm, spent hydrogen stream (85) is vented or usedelsewhere in the plant whereas the second portion of the warm, usedhydrogen gas (86) is recycled to the second heat exchanger (59) using anauxiliary blower (87). This warm, used hydrogen gas (86) is re-cooled inthe second heat exchanger (59) to a temperature of about −147° C. usingthe cold nitrogen gas (66). The re-cooled hydrogen gas (88) is thencombined with the cold, spent hydrogen gas (80) from the separator (57).The combined hydrogen stream (82) is directed to the economizer (53) tocool the influent or feed process stream (52).

From the foregoing, it should be appreciated that the present inventionthus provides an improved method and system for cryogenic cooling of aprocess stream. While the invention herein disclosed has been describedby means of specific embodiments and processes associated therewith,numerous modifications and variations can be made thereto by thoseskilled in the art without departing from the scope of the invention asset forth herein or sacrificing all its material advantages.

What is claimed is:
 1. A method for cryogenic cooling of a processstream comprising the steps of: pre-chilling an influent of warm processgas in an economizer; cooling the pre-chilled process gas with a cryogenin a cryogenic heat exchanger to a prescribed final temperature;separating the cooled process gas at the prescribed final temperatureinto a condensable product and a cold spent process gas; recycling thecold spent process gas to the economizer to pre-chill the influent ofwarm process gas; forcibly directing a portion of the used process gasrecycled to the economizer to an auxiliary heat exchanger; and directingthe spent cryogen from the cryogenic heat exchanger to the auxiliaryheat exchanger to re-cool the used process gas; wherein the excessrefrigeration capacity of the cold spent process gas is directlytransferred to the influent warm process gas flowing through theeconomizer and the excess refrigeration capacity of the spent cryogen isindirectly transferred to the influent warm process gas flowing throughthe economizer; and wherein the amount of cryogen needed to cool thepre-chilled process gas in the cryogenic heat exchanger to a prescribedtemperature is minimized.
 2. The method of claim 1 wherein the cryogeniccooling is applied to a polysilicon manufacturing process and whereinthe warm process gas is a gaseous stream of silane in hydrogen, thecryogen is nitrogen in both liquid and gaseous form, the condensableproduct is liquid silane at temperatures of lower than about −173° C.and the cold spent process gas is hydrogen gas at temperatures lowerthan about −172° C.
 3. A cryogenic cooling system comprising: aninfluent process stream; a source of cryogen; a cryogenic heat exchangerfor cooling the process stream using the cryogen; a phase separatordisposed downstream of the cryogenic heat exchanger, the phase separatoradapted for separating the cooled process stream into a condensableproduct and a cold spent process gas; an economizer for pre-chilling theinfluent process stream with the cold spent process gas, the economizerdisposed upstream of the cryogenic heat exchanger; a first recycleconduit coupling the outlet of the phase separator to the economizer todirect the cold spent process gas from the phase separator to theeconomizer to pre-chill the influent process stream; a second heatexchanger coupled to the cryogenic heat exchanger and adapted for usingspent cryogen from the cryogenic heat exchanger to cool a stream of usedprocess gas recycled to the economizer; a second recycle conduitcoupling the outlet of the economizer through the second heat exchangerand to either the first recycle conduit or the inlet of the economizerto pre-chill the influent process stream; a blower disposed in operativeassociation with the second recycle conduit to forcibly drive the usedprocess gas from the outlet of the economizer through the second heatexchanger to either the first recycle conduit or the inlet of theeconomizer: wherein excess refrigeration capacity of the spent cryogenfrom the first heat exchanger is transferred first to the used processgas flowing through the second heat exchanger and subsequently to theinfluent process stream flowing through the economizer; and whereinexcess refrigeration capacity of the cold spent process gas exiting thephase separator is transferred to the influent process stream flowingthrough the economizer.
 4. The cryogenic cooling system of claim 3wherein the cryogenic cooling system is integrated in a polysiliconmanufacturing process and wherein the process gas is a gaseous stream ofsilane in hydrogen, the cryogen is nitrogen in both liquid and gaseousform, the condensable product from the phase separator is liquid silaneand the cold spent process gas from the phase separator is hydrogen gas.5. An improvement to a cryogenic cooling system comprising a cryogenicheat exchanger for cooling a process stream; a phase separatordownstream of the cryogenic heat exchanger for separating the cooledprocess stream into a condensable product and a cold spent process gas,and an economizer for pre-chilling the process stream upstream of thecryogenic eat exchanger, the improvement further comprising: a secondheat exchanger coupled to the cryogenic heat exchanger; a first recycleconduit coupling the outlet of the phase separator to the economizer todirect the cold spent process gas from the phase separator to theeconomizer; a second recycle conduit coupling the outlet of theeconomizer through the second heat exchanger and to either the firstrecycle conduit or the inlet of the economizer; a blower disposed inoperative association with the second recycle conduit to forcibly driveused process gas from the outlet of the economizer through the secondheat exchanger and to either the first recycle conduit or the inlet ofthe economizer; wherein excess refrigeration capacity of the spentcryogen exiting the first heat exchanger is transferred to the usedprocess gas flowing through the second heat exchanger subsequently tothe influent process stream flowing through the economizer; and whereinthe excess refrigeration capacity of the cold spent process gas exitingthe phase separator is transferred directly to the influent processstream flowing through the economizer.
 6. The improvement of claim 5wherein the cryogenic cooling system is integrated in a polysiliconmanufacturing process and wherein the process gas is a gaseous stream ofsilane in hydrogen, the cryogen is nitrogen, the condensable productfrom the phase separator is liquid silane and the cold spent process gasfrom the phase separator is hydrogen gas.